Conventionally, in a thin-film solar battery, by adopting a tandem structure having a film including a crystalline layer in a power generation layer, light in a wide wavelength range of sunlight is absorbed, thereby improving the photoelectric conversion efficiency. To further increase optical absorption in a wide wavelength range of sunlight, an optical confinement technique is applied. As the optical confinement technique, when light enters from a side of a translucent insulating substrate, a method of forming an uneven structure on a surface of a transparent conductive film on the translucent insulating substrate is used.
Regarding the technique for forming the uneven structure, it has been generally known that the photoelectric conversion efficiency of the thin-film solar battery is improved due to a reduction effect of optical reflectivity and a light scattering effect thereof. More specifically, light incident from the side of the translucent insulating substrate is scattered in an interface between the transparent conductive film having the uneven structure and the power generation layer, and then enters into the power generation layer, and thus light enters into the power generation layer substantially diagonally. As the light enters into the power generation layer diagonally, a substantial optical path of light in the power generation layer is extended to increase optical absorption, and thus an output current of the solar battery increases.
conventionally, as a transparent conductive film having such an uneven structure, tin oxide (SnO2) has been well known. Generally, the uneven structure on a surface of tin oxide (SnO2) is formed by causing a growth of crystal grains having a diameter of several tens of nanometers to several micrometers on the film surface by a thermal chemical vapor deposition (CVD) method.
On the other hand, from a viewpoint of being excellent in plasma-resistance and abundant in resource wise, zinc oxide (ZnO) has been prevalent as a material of the transparent conductive film as an alternative to tin oxide (SnO2). When zinc oxide (ZnO) is used as the material of a transparent conductive film, there has been proposed a technique such that a transparent conductive film is first formed on a glass substrate by a sputtering method, and then the transparent conductive film is etched by using acid, thereby forming an uneven structure on a film surface (see, for example, Patent Literature 1). By easily forming an uneven structure using this method, cost reduction is expected in manufacturing thin-film solar batteries.
However, according to the technique of Patent Literature 1 described above, because an uneven structure with steep slopes is formed on a surface of the transparent conductive film by etching, there is a problem that a defect can occur in a power generation layer due to the uneven structure with steep slopes, and this causes deterioration of the yield and reliability. Therefore, in recent years, to achieve a high light scattering performance in a wider wavelength range of sunlight, such a structure having a more microscopic uneven structure on a macroscopic uneven structure on a surface of a transparent conductive film has been proposed (see, for example, Patent Literature 2).